1. Field of the Invention
The present invention relates to atomizing devices and to methods of making the same and, more particularly, to gas-assisted, micromachined, atomizing devices that produce small droplets and to methods of making the same.
2. Description of the Related Art
Liquid atomizing devices are used in various mechanisms, such as medical nebulizers and fuel injectors for combustion chambers. The performance of many of these mechanisms can be improved if the atomizing device provides a spray with very small droplets. For example, small droplets improve the effectiveness of medical nebulizers because small droplets (e.g., between 2 and 5 micrometers) can be inhaled deep into the lungs. Additionally, small droplets (e.g., less than 20 micrometers) improve the efficiency of combustion devices by causing faster vaporization of the fuel.
Conventional atomizing devices typically provide a spray having droplets within a wide range of sizes, including a small percentage of droplets that have a Sauter mean diameter smaller than 10 micrometers. Conventional atomizing devices have rarely been able to provide a spray having droplets limited to a small range of sizes and having a Sauter mean diameter smaller than 10 micrometers, without employing additional mechanisms such as ultrasonic power or high-voltage electrostatic charging.
The failure of conventional atomizing devices to provide a small range and small droplets can be attributed to the manner in which these devices perform atomization. Conventional atomizing devices break bulk liquid into relatively large ligaments, break the ligaments into relatively large drops through atomization, and break the large drops into smaller droplets through secondary atomization. As the droplets become smaller than 100 micrometers, they become harder to break, and secondary atomization typically ceases, thus preventing most of the droplets from becoming as small as 10 micrometers. Also, since the bulk liquid is much larger than the desired droplet size and, therefore, must be broken down a number of times to become relatively small, the droplets ultimately formed by conventional devices will have a relatively wide size range.
Efforts have been made to decrease droplet size by increasing the amount of gas forced through the atomizing device. However, this results in a large gas-liquid mass ratio, which is undesirable for many applications because it requires a large gas pump, a large amount of gas, and a high gas velocity.
Another problem associated with conventional atomizing devices is that two devices, even of the same type, often will have different spray characteristics. These differing spray characteristics result from very minor variations in the structure of the atomizing device. With current manufacturing methods, these variations occur more frequently than is desired.
An object of the present invention is to provide an atomizing devices that solve the foregoing problems.
Another object of the present invention is to provide atomizing devices that produce a spray having droplets with a Sauter mean diameter of 10 micrometers or smaller.
Yet another object of the present invention is to provide atomizing devices that produce a spray having droplets within a small range of diameters.
Yet another object of the present invention is to provide atomizing devices having a small gas-liquid mass ratio.
Yet another object of the present invention is to provide atomizing devices of very small size.
Yet another object of the present invention is to provide atomizing devices that can be mass produced and that, nevertheless, have consistent spray characteristics from device to device.
Additional objects and advantages of the invention will become apparent from the description which follows. Additional advantages may also be learned by practice of the invention.
In a broad aspect, the invention provides a method of atomizing a liquid, comprising the steps of flowing a liquid over an atomizing edge of an orifice, and flowing a gas against the liquid to cause atomization of the liquid into droplets having a Sauter mean diameter smaller than 35 micrometers at a gas-liquid mass ratio of less than or equal to 0.2.
In another broad aspect, the invention provides a method of atomizing a liquid, comprising the steps of flowing a liquid over an atomizing edge of an orifice, and flowing a gas against the liquid to cause primary atomization of the liquid into droplets having a Sauter mean diameter smaller than a critical diameter Dmax of the droplets, where:
Dmax=8"sgr"/(CDxcfx81AUR2)
where:
"sgr": surface tension of the liquid;
CD: drag coefficient of a droplet having a diameter equal to the critical diameter;
xcfx81A: density of the gas; and
UR: relative velocity between the droplet and the gas.
In another broad aspect, the invention provides an atomizing device comprising a substantially planar first layer having a first opening therethrough, and a substantially planar second layer having a second opening therethrough and being laminated to the first layer such that the first and second openings are aligned to form a main gas orifice that guides a main gas in a flow direction, the second opening being bounded by at least one inner surface with at least one atomizing edge, wherein the first and second layers define at least one liquid orifice that supplies liquid to be atomized onto the at least one inner surface of the second layer where the liquid forms a thin film.
In another broad aspect, the invention provides a method of forming an atomizing device, comprising the steps of forming a first opening in a substantially planar first layer, forming a second opening in a substantially planar second layer, the second opening having at least one inner surface with an atomizing edge, forming at least one liquid orifice in at least one of the first and second layers, and connecting the first and second layers such that the first and second openings are aligned to form a main gas orifice that guides a main gas in a flow direction and such that the liquid orifice supplies liquid to be atomized onto the at least one inner surface of the second opening.
In another broad aspect, the invention provides a gas-assisted atomizing device comprising a substantially planar first layer, and a substantially planar second layer having a plurality of orifices formed therein, wherein the first and second layers form a gas supply network including a plurality of gas channels that supply gas to at least some the plurality of orifices, and a liquid supply network including a plurality of liquid channels that supply liquid to at least some of the plurality of orifices.
In another broad aspect, the invention provides a method of forming a gas-assisted atomizing device, comprising the steps of forming a gas supply network and a liquid supply network in a substantially planar first layer and a substantially planar second layer, forming a plurality of orifices in the second layer for releasing a spray, and connecting the first and second layers such that the gas and liquid supply networks supply gas and liquid to form a spray at the plurality of orifices.
In another broad aspect, the invention provides a gas-assisted atomizing device comprising a substantially planar first layer, and a substantially planar second layer having a plurality of liquid orifices and a plurality of gas orifices formed therein. The first and second layers form a liquid supply network including a plurality of liquid channels that supply liquid to the plurality of liquid orifices and force liquid through the liquid orifices to form streams of liquid, and a gas supply network including a plurality of gas channels that supply gas to the plurality of gas orifices and force gas through the gas orifices to atomize the streams of liquid.
It is to be understood that both the foregoing summary and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.